There is an increasing demand on mobile wireless operators to provide voice and high-speed data services, and at the same time, these operators want to support more users per basestation to reduce overall network costs and make the services affordable to subscribers. As a result, wireless systems that enable higher data rates and higher capacities are needed. The available spectrum for wireless services is limited, however, and the prior attempts to increase traffic within a fixed bandwidth have increased interference in the system and degraded signal quality.
Wireless communications networks are typically divided into cells, with each of the cells further divided into cell sectors. A base station is provided in each cell to enable wireless communications with mobile stations located within the cell. One problem exists when prior art omni-directional antennas are used at the basestation because the transmission/reception of each user's signal becomes a source of interference to other users located in the same cell location on the network, making the overall system interference limited. Such an omni-directional antenna is shown in FIG. 1(a).
In these traditional omni-directional antenna cellular network systems, the base station has no information on the position of the mobile units within the cell and radiates the signal in all directions within the cell in order to provide radio coverage. This results in wasting power on transmissions when there are no mobile units to reach, in addition to causing interference for adjacent cells using the same frequency, so called co-channel cells. Likewise, in reception, the antenna receives signals coming from all directions including noise and interference.
An effective way to increase efficiency of bandwidth usage and reduce this type of interference is to use multiple input-multiple output (MIMO) technology that supports multiple antennas at the transmitter and receiver. For a multiple antenna broadcast channel, such as the downlink on a cellular network, transmit/receive strategies have been developed to maximize the downlink throughput by splitting up the cell into multiple sectors and using sectorized antennas to simultaneously communicate with multiple users. Such sectorized antenna technology offers a significantly improved solution to reduce interference levels and improve the system capacity.
The sectorized antenna system is characterized by a centralized transmitter (cell site/tower) that simultaneously communicates with multiple receivers (user equipment, cell phone, etc.) that are involved in the communication session. With this technology, each user's signal is transmitted and received by the basestation only in the direction of that particular user. This allows the system to significantly reduce the overall interference in the system. A sectorized antenna system, as shown in FIG. 1(b), consists of an array of antennas that direct different transmission/reception beams toward users located in the coverage area of the sector of the cell.
To improve the performance of a sectorized cell sector, schemes have been implemented using orthogonal frequency domain multiple access (OFDMA) systems, which are also called Space-Division Multiple Access (SDMA) systems. In these systems, mobile stations can communicate with the base station using one or more of these spatial beams. This method of orthogonally directing transmissions and reception of signals, called beamforming, is made possible through advanced signal processing at the base station.
A beamforming scheme is defined by the formation of multiple spatial beams within a cell sector to divide the cell sector into different coverage areas. The radiation pattern of the base station, both in transmission and reception, is adapted to each user to obtain highest gain in the direction of that user. By using sectorized antenna technology and by leveraging the spatial location and channel characteristics of mobile units within the cell, communication techniques called space-division multiple access (SDMA) have been developed for enhancing performance. Space-Division Multiple Access (SDMA) techniques essentially creates multiple, uncorrelated spatial pipes transmitting simultaneously through beamforming and/or precoding, by which it is able to offer superior performance in multiple access radio communication systems.
One type of beamforming scheme is an adaptive beamforming scheme that dynamically directs beams toward a location of a mobile station. Such an adaptive beamforming scheme requires mobility tracking in which locations and spatial characteristics of mobile stations are tracked for the purpose of producing the adaptive beams. Depending on location and spatial characteristics, each user's signal is multiplied by complex weightings that adjust the magnitude and phase of the signal to and from each antenna. This causes the output from the array of sectorized antennas to form a transmit/receive beam in the desired direction and minimizes the output in other directions, which can be seen graphically in FIG. 2.
However, the mobility and spatial channel tracking of the user's location in the network cell required by these beamforming antenna systems increases the overhead of the system. Moreover, mobility and spatial channel tracking may not be possible or practical with mobile stations moving at relatively high velocities. There is a need for support of sectorized beamforming antenna systems in mobile broadband communication networks, including solving some of the above-identified problems.
The various components on the system may be called different names depending on the nomenclature used on any particular network configuration or communication system. For instance, “user equipment” encompasses PC's on a cabled network, as well as other types of equipment coupled by wireless connectivity directly to the cellular network as can be experienced by various makes and models of mobile terminals (“cell phones”) having various features and functionality, such as Internet access, e-mail, messaging services, and the like.
Further, the words “receiver” and “transmitter” may be referred to as “access point” (AP), “basestation,” and “user” depending on which direction the communication is being transmitted and received. For example, an access point AP or a basestaion (eNodeB or eNB) is the transmitter and a user is the receiver for downlink environments, whereas an access point AP or a basestaion (eNodeB or eNB) is the receiver and a user is the transmitter for uplink environments. These terms (such as transmitter or receiver) are not meant to be restrictively defined, but could include various mobile communication units or transmission devices located on the network.